An odorant binding protein is involved in counteracting detection-avoidance and Toll-pathway innate immunity

Introduction: Odorant-binding proteins (OBPs) are a class of small molecular weight soluble proteins that exist as expanded gene families in all insects, acting as ligand carriers mediating olfaction and other physiological processes. During fungal infection, a subset of insect OBPs were shown to be differentially expressed. Objectives: We tested whether the altered expression of insect OBPs during pathogenic infection plays a role in behavioral or immune interactions between insect hosts and their pathogens. Methods: A wide range of techniques including RNAi-directed knockdown, heterologous protein expres-sion, electrophysiological/behavioral analyses, transcriptomics, gut microbiome analyses, coupled with tandem mass spectrometry ion monitoring, were used to characterize the function of a locust OBP in host behavioral and immune responses. Results: The entomopathogenic fungus Metarhizium anisopliae produces the volatile compound pheny-lethyl alcohol (PEA) that causes behavioral avoidance in locusts. This is mediated by the locust odorant binding protein 11 (LmOBP11). Expression of LmOBP11 is induced by M. anisopliae infection and PEA treat-ment. LmOBP11 participates in insect detection of the fungal-produced PEA and avoidance of PEA -contaminated food, but the upregulation of LmOBP11 upon M. anisopliae infection negatively affects the insect immune responses to ultimately benefit successful mycosis by the pathogen. RNAi knockdown of LmOBP11 increases the production of antimicrobial peptides and enhances locust resistance to M. anisopliae infection, while reducing host antennal electrophysiological responses to PEA and locust avoid-ance of PEA treated food. Also, transcriptomic and gut microbiome analyses reveal microbiome dysbiosis and changes in host genes involved in behavior and immunity. These results are consistent with the ele-vated expression of LmOBP11 leading to enhanced volatile detection and suppression of immune responses. Conclusion: These findings suggest a crosstalk between olfaction and immunity, indicating manipulation of host OBPs as a novel target exploited by fungal pathogens to alter immune activation and thus promote the successful infection of the host. & COPY; 2023 The Authors. Published by Elsevier B.V. on behalf of Cairo University. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Automated summary

This study investigates the impact of altered expression of insect odorant-binding proteins (OBPs) during fungal infection on host behavior and immune responses. Through various techniques, it was found that the upregulation of OBP LmOBP11 in response to fungal infection affects insect behavior and immune activation. LmOBP11 is involved in insect detection of fungal volatile compounds and avoidance of contaminated food, but its elevated expression suppresses immune responses, promoting successful fungal infection. RNAi knockdown of LmOBP11 enhances host resistance and antimicrobial peptide production.